Nonwoven spunbonded fabrics are used in many applications and account for the majority of products produced or used in North America. Almost all such applications require a lightweight disposable fabric. Therefore, most spunbonded fabrics are designed for single use and are designed to have adequate properties for the applications for which they are intended. Spunbonding refers to a process where the fibers (filaments) are extruded, cooled, and drawn and subsequently collected on a moving belt to form a fabric. The web thus collected is not bonded and the filaments must be bonded together thermally, mechanically, or chemically to form a fabric. Thermal bonding is by far the most efficient and economical means for forming a fabric. Hydroentangling is not as efficient, but leads to a much more flexible and normally stronger fabric when compared to thermally bonded fabrics.
Microdenier fibers are fibers which are smaller than 1 denier. Typically, microdenier fibers are produced utilizing a bicomponent fiber which is split. FIG. 1 illustrates the best know type of splittable fiber commonly referred to as “pie wedge” or “segmented pie.” U.S. Pat. No. 5,783,503 illustrates a typical meltspun muticomponent thermoplastic continuous filament which is split absent mechanical treatment. In the configuration described, it is desired to provide a hollow core filament. The hollow core prevents the tips of the wedges of like components from contacting each other at the center of the filament and promotes separation of the filament components.
In these configurations, the components are segments typically made from nylon and polyester. It is common for such a fiber to have 16 segments. The conventional wisdom behind such a fiber has been to form a web of typically 2 to 3 denier per filament fibers by means of carding and/or airlay, and subsequently split and bond the fibers into a fabric in one step by subjecting the web to high pressure water jets. The resultant fabric will be composed of microdenier fibers and will possess all of the characteristics of a microdenier fabric with respect to softness, drape, cover, and surface area.
When manufacturing bicomponent fibers for splitting, several characteristics of the fibers are typically required for consideration to ensure that the continuous fiber may be adequately manufactured. These characteristics include the miscibility of the components, differences in melting points, the crystallization properties, viscosity, and the ability to develop a triboelectric charge. The copolymers selected are typically done to ensure that these characteristics between the bicomponent fibers are accommodating such that the muticomponent filaments may be spun. Suitable combinations of polymers include polyester and polypropylene, polyester and polyethylene, nylon and polypropylene, nylon and polyethylene, and nylon and polyester. Since these bicomponent fibers are spun in a segmented cross-section, each component is exposed along the length of the fiber. Consequently, if the components selected do not have properties which are closely analogous, the continuous fiber may suffer defects during manufacturing such as breaking or crimping. Such defects would render the filament unsuitable for further processing.
U.S. Pat. No. 6,448,462 discloses another muticomponent filament having an orange-like multisegment structure representative of a pie configuration. This patent also discloses a side-by-side configuration. In these configurations, two incompatible polymers such as polyesters and a polyethylene or polyamide are utilized for forming a continuous muticomponent filament. These filaments are melt-spun, stretched and directly laid down to form a nonwoven. The use of this technology in a spunbond process coupled with hydro-splitting is now commercially available as a product marketed under the EVOLON® trademark by Freudenberg and is used in many of the same applications described above.
The segmented pie is only one of many possible splittable configurations. In the solid form, it is easier to spin, but in the hollow form, it is easier to split. To ensure splitting, dissimilar polymers are utilized. But even after choosing polymers with low mutual affinity, the fiber's cross section can have an impact on how easily the fiber will split. The cross section that is most readily splittable is a segmented ribbon, such as that shown in FIG. 2. The number of segments has to be odd so that the same polymer is found at both ends so as to “balance” the structure. This fiber is anisotropic and is difficult to process as a staple fiber. As a filament, however, it would work fine. Therefore, in the spunbonding process, this fiber can be attractive. Processing is improved in fibers such as tipped trilobal or segmented cross. See FIG. 3.
Another disadvantage utilizing segmented pie configurations is that the overall fiber shape upon splitting is a wedge shape. This configuration is a direct result of the process to producing the small microdenier fibers. Consequently, while suitable for their intended purpose, nonetheless, other shapes of fibers may be desired which produce advantageous application results. Such shapes are currently unavailable under standard segmented processes.
Accordingly, when manufacturing microdenier fibers utilizing the segmented pie format, certain limitations are placed upon the selection of the materials utilized and available. While the components must be of sufficiently different material so the adhesion between the components is minimized facilitating separation, they nonetheless also must be sufficiently similar in characteristics in order to enable the fiber to be manufactured during a spunbond or meltblown process. If the materials are sufficiently dissimilar, the fibers will break during processing.
Another method of creating microdenier fibers utilizes fibers of the island in the sea configuration. U.S. Pat. No. 6,455,156 discloses one such structure. In an island in the sea configuration, a primary fiber component, the sea, is utilized to envelope smaller interior fibers, the islands. Such structures provide for ease of manufacturing, but require the removal of the sea in order to reach the islands. This is done by dissolving the sea in a solution which does not impact the islands. Such a process is not environmentally friendly as an alkali solution is utilized, which requires waste water treatment. Additionally, since it is necessary to extract the island components, the method restricts the types of polymers which may be utilized in that they are not affected by the sea removal solution.
Such island in the sea fibers are commercially available today. They are most often used in making synthetic leathers and suedes. In the case of synthetic leathers, a subsequent step introduces coagulated polyurethane into the fabric, and may also include a top coating. Another end-use that has resulted in much interest in such fibers is in technical wipes, where the small fibers lead to a large number of small capillaries resulting in better fluid absorbency and better dust pick-up. For a similar reason, such fibers may be of interest in filtration.
In summary, what has been accomplished so far has limited application because of the limitations posed by the choice of the polymers that would allow ease of spinning and splittability for segmented fibers. The spinning is problematic because both polymers are exposed on the surface and therefore, variations in elongational viscosity, quench behavior, and relaxation cause anisotropies that lead to spinning challenges. Further, a major limitation of the current art is that the fibers form wedges and there is no flexibility with respect to fiber cross sections that can be achieved.
An advantage with an island in the sea technology is that if the spinpack is properly designed, the sea can act as a shield and protect the islands so as to reduce spinning challenges. However, with the requirement of removing the sea, limitations upon the availability of suitable polymers for the sea and island components are also restricted. Heretofore, islands in the sea technology is not employed for making microdenier fibers other than via the removal of the sea component because of the common belief that the energy required to separate the islands from the sea is not commercially viable.
Accordingly, there is a need for a manufacturing process which can produce microdenier fibers dimensions in a manner which is conducive to spunbound processing and which is environmentally sound.